I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for controlling transmission power in a wireless communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Pat. No. 5,414,796, entitled xe2x80x9cVARIABLE RATE VOCODERxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when said speech encoding is providing speech data at a maximum rate. When a variable rate speech coder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate is described in detail in U.S. Pat. No. 5,504,773, entitled xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSIONxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Pat. No. 5,659,569, entitled xe2x80x9cDATA BURST RANDOMIZERxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In a communication system that provides data using a QPSK modulation format, very useful information can be obtained by taking the cross product of the I and Q components of the QPSK signal. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled xe2x80x9cPILOT CARRIER DOT PRODUCT CIRCUITxe2x80x9d, assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in copending U.S. patent Ser. No. 08/886,604, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The present invention is a novel and improved power control system for use in a communication system in which the transmission energy may be gated or capped or closed loop power control commands otherwise ignored without the knowledge of the transmitter of those power control commands. The present invention is illustrated in the context of controlling forward link transmission power. It will be understood by one skilled in the art that the transmission present invention is equally applicable to controlling the reverse link transmission power and that the invention is in no way limited to the application of forward link power control. Simply, by exchanging the references of the mobile station to references of a base station and changing references of the base station to references to the mobile station, a reverse link power control system is described.
In mobile station 7, the forward link signals 3 are received and demodulated. In addition, the mobile station determines the adequacy of the received signal power of forward link signals 3. In accordance with the determined adequacy of the received signal energy of forward link signals 3, mobile station 7 generates a series power control commands. In the exemplary embodiment, the power control commands consist of a series of simple up/down commands to which base station 1 responds by increasing or decreasing the transmission energy of forward link signals 3. The present invention is equally applicable to the generation of other forms of closed loop power control, such as the generation of power control commands that are indicative of the amount of change to the transmission power of forward link signals 3.
In the exemplary embodiment, power control commands are generated by comparing the received signal to interference ratio (SIR) of forward link signals 3 to at least one signal to interference ratio threshold. In the exemplary embodiment, a single signal to interference ratio threshold is used in the generation of a one bit power control command. The signal to energy ratio threshold is set to provide a desired performance level, such as a desired frame error rate or symbol error rate. This desired performance level may vary based upon the type of service being provided on forward link signal 3. When the performance level of the received forward link signal 3 varies from the desired performance level, the signal to interference ratio threshold is changed.
If the performance level of the received signal is less than the desired performance level, then the signal to interference ratio threshold is increased which will result in an increase in the received energy of forward link signals 3. Conversely, if the performance level of the received signal is greater than the desired performance level, then the signal to interference ratio threshold is decreased. It may seem counter intuitive to speak of a received signal being of too great a quality, but it should be remembered that this excess quality represents unnecessary energy employed in the transmission of forward link signals 3, which results in degradation of service to all other mobile stations served by base station 1 and to a reduction in the number of mobile station capable of being served by base station 1.
The varying of the signal to interference ratio thresholds based on measured performance metrics is referred to as outer loop power control. The feedback of power control commands based on comparing the measured signal to interference ratio to the variable threshold is referred to as closed loop power control. The combination of closed loop and outer loop power control is contemplated in both the ETSI UTRA candidate submission and the TIA cdma2000 candidate submission. The combination of closed loop power control and open loop power control is described in detail in aforementioned U.S. Pat. No. 5,056,109.
Mobile station 7 generates the power control commands and transmits them along with traffic data, pilot symbol data back to base station 1 on reverse link signals 5. In the exemplary embodiment, reverse link signals 5 are CDMA signals. In particular, in the exemplary embodiment, reverse link signals 5 are CDMA signals transmitted in accordance with the description in both the ETSI UTRA candidate submission and the TIA cdma2000 candidate submission. The reverse link signals for these two submissions are essentially identical for the purposes of the present invention. It should be understood that the present invention is equally applicable to other forms of CDMA signals and to other modulation schemes such as TDMA or GSM modulation schemes.
Base station 1 receives the power control commands from mobile station 7, and in response to those commands increases or decreases the transmission energy of forward link signals 3. However, there may be times when base station 1 does not respond to the power control commands from mobile station 7. For example, base station 1 may not increase the energy of forward link signals 3 in response to power control commands from mobile station 7, when base station 1 determines that it cannot allocate additional energy for the transmission to forward link signals 3 without causing unacceptable degradation to the transmission of signals to other mobile stations served by base station 1.
When base station 1 does not respond to the power control commands from mobile station 7, it can either continue transmitting at the present transmission energy of forward link signals 3 (referred to herein in as xe2x80x9ccappingxe2x80x9d the energy of forward link signals 3) or it can temporarily gate off the transmission of forward link signals 3 (referred to herein in as xe2x80x9cgatingxe2x80x9d the energy of forward link signals 3). When the energy of forward link signals 3 is either capped or gated, reception of those frames or symbols transmitted at the capped or gated energy in error may be unreliable.
Under traditional implementations, in response to the reception of the capped or gated frames or symbols, mobile station 7 will erroneously increase its signal to interference ratio threshold in the outer loop power control adjustment described above. This adjustment must be inhibited because the cause of the frame or symbols errors is not a result of mobile station 7 sending incorrect power control commands but rather because base station 1 is ignoring those commands and this is not within the control of mobile station 7. The present invention addresses this problem of controlling adjustment of the outer loop power control system in the presence of potentially gated or capped signals.